In recent years, light emitting devices such as light emitting diode (LED) are put into practical use in various display boards, image reading light sources, traffic signals, large-sized display units, backlights of mobile phones, and the like. Those light emitting devices are generally encapsulated with a curable resin obtained by curing an aromatic epoxy resin and an alicyclic acid anhydride as a curing agent.
However, it is known that the aromatic epoxy resin system has the problems such that the alicyclic acid anhydride is easy to discolor with an acid, and it takes long time for the resin to be cured. Furthermore, the aromatic epoxy resin system has the problem such that a curable resin used for encapsulation turns yellow when a light emitting device is placed outdoor or is exposed to a light source generating ultraviolet rays.
To overcome those problems, a method for encapsulating LED and the like with a curable resin using an alicyclic epoxy resin or an acrylic resin, and a cationic polymerization initiator is attempted (see Patent Documents 1 and 2).
However, the cationically polymerized curable resin is very brittle and has the disadvantage that crack destruction occurs easily by a thermal cycle test (also called a heat cycle test). Furthermore, this curable resin has the disadvantage that coloration of the curable resin used for encapsulation after curing is remarkable as compared with the conventional curable resin using an aromatic epoxy resin and an acid anhydride. For this reason, the curable resin is not suitable for use which requires colorless transparency, especially use for LED encapsulation which requires heat resistance and transparency.
A resin composition for an encapsulation material of LED which is improved in occurrence of crack destruction by a thermal cycle test and which has excellent light resistance is investigated (see Patent Document 3). The resin composition disclosed in Patent Document 3 uses a hydrogenated epoxy resin and an alicyclic epoxy resin as matrix components, but since coloration after curing is considerable, the improvement in discoloration is still desired.
On the other hand, white LED is used for use such as illumination, and with increasing its output, generation of heat in a LED package cannot be disregarded. When an epoxy resin is used as an encapsulation material, yellowing due to its generation of heat cannot be prevented. For this reason, in place of an epoxy resin, a silicone resin has been used as an encapsulation material of white LED.
The silicone resin used in LED is roughly classified into two categories, namely, a phenyl silicone resin and a methyl silicone resin. The phenyl silicone resin which is generally used meets a refractive index. On the other hand, the phenyl silicone resin is excellent in heat resistance as compared with the epoxy resin, but is not yet sufficient to meet a high LED output.
Although the other resin, the methyl silicone resin, has very excellent heat resistance and weather resistance, the methyl silicone resin has the disadvantage that light extraction efficiency of LED is poor since it has low refractive index. Furthermore, the cured methyl silicone resin is very brittle, and has the disadvantage that crack destruction occurs easily by a thermal cycle test, and the disadvantage that adhesiveness to a polyamide resin used in an LED substrate is weak as compared with an epoxy resin and the like.
In view of the above, an encapsulation material that can meets large output of white LED and achieve a good balance of properties among high refractive index, heat resistance and the like, and a thermosetting composition used in the material are required.
On the other hand, Patent Documents 4 to 8 each disclose a cage-like silicon compound and its polymer. However, those are a solid or a crystal. Therefore, to apply to the uses of molding of LED and the like, a solvent for dissolving those is required.